It is assumed that an ASK signal generator is set on when the amplitude of a sine wave is large and is set off when the amplitude of the sine wave is extremely small. In the off state, it is ideal to set a state in which the amplitude of the sine wave is zero, but in practice, the amplitude of the sine wave is output because of leakage. Therefore, as one of indices indicating the quality of an ASK signal, the on/off ratio indicating the ratio of ASK modulation output amplitudes in the on and off states is provided. For example, it is required to realize the on/off ratio of, for example, approximately 15 dB to correctly demodulate an ASK signal by means of a receiver.
In a general ASK signal generator, the on/off ratio is set in a tradeoff relationship with respect to the insertion loss of an ASK modulator. When the ASK signal generator is designed, the optimum design is made to permit two characteristics of the on/off ratio and the insertion loss to satisfy desired specifications.
Further, when signal power dealt with by the ASK signal generator becomes large, the ASK modulator performs a nonlinear operation with a large input signal, output power at the on time decreases and, at the same time, leakage power at the off time increases to degrade the on/off ratio. A target on/off ratio can be achieved when input power to the ASK modulator is sufficiently small, but the on/off ratio is degraded as the input signal increases and it becomes impossible to satisfy the target specification. Therefore, it is desired to develop an ASK signal generator that can enhance the on/of ratio.